The invention applies notably, but not restrictively, to memories that are capable of operating over a wide range of power supply voltages including low voltages (for example between 1.6 V and 5.5 V).
The use of a power on reset circuit, commonly denoted by “POR”, is prevalent in memory devices of EEPROM type.
A POR circuit generates a reset signal, which is notably applied to the registers of the functional circuit with which it is associated, when the power supply voltage reaches a specified minimum operating value. This makes it possible to ensure that the functional circuit begins to operate in a known state.
It is likewise known that a POR circuit can be used as a protection circuit by detecting a drop in power supply voltage below a threshold level.
Such a circuit is advantageously used on memory devices of EEPROM type, avoiding incorrect reading or memory storage of corrupt data when the power supply falls below a voltage that is referred to as a malfunction voltage.
Thus, the threshold level of a POR circuit needs to be above the malfunction voltage and below the specified minimum operating voltage of the memory.
However, the threshold level of a POR circuit can vary by a few hundred millivolts, notably because of manufacturing constraints, temperature or ageing of the components. Thus, the effective threshold level of a POR circuit is generally in a range limited by an upper limit and a lower limit.
To avoid malfunctions, it is recommended that the level of the lower limit be above the malfunction voltage, and to avoid untimely triggering of a “reset” signal, it is recommended that the level of the upper limit be below the minimum operating voltage.
This condition can give rise to difficulties when the margin between the specified minimum operating voltage and the malfunction voltage is narrower than the range defined by the limits of the effective threshold level of the POR.
This is because in such a case, and in accordance with the variations in the effective threshold level, it will be possible for the POR not to detect a power supply voltage level below the malfunction voltage. It will likewise be possible for the POR to trigger “reset” signals during normal operation.
By way of example, and without restriction, a non-volatile memory device can have a specified minimum operating voltage at 1.5 V, and a first malfunction owing to a fall in voltage to 1.4 V. If the threshold level of the POR can vary in a range of 200 mV, by way of example, it will then not be possible in this case to have both the upper limit of the threshold level below the minimum operating voltage and the lower limit of the threshold level above the malfunction voltage.
Furthermore, the lower limit of the POR circuit can be correct for ensuring the power on reset but can be too low for preventing malfunctions.
By way of example, a memory operating at 20 MHz, under 5 V risks suffering malfunctions if data are interchanged at 20 MHz in the event of a slow drop in voltage from the value of 5 V. Moreover, such a malfunction will not be able to be prevented by a POR circuit.